Recently, it has been desired to highly increase the sensitivity of silver halide photographic light-sensitive materials, in particular, used for photography, as typically shown by ISO 400 films, etc. In order to increase the sensitivity, investigations have been made on a variety of techniques, including, e.g., use of large size silver halide grains, couplers with higher activities, accelerated development, etc. However, the limit appears to have been reached in increasing the sensitivity based on use of large size silver halide grains, as reported by G. C. Farnell and J. B. Chanter in Journal of Photographic Science, Vol. 9, page 75 (1961). Accordingly, this technique is not expected to make much of a contribution in the future. In addition, the use of large size silver halide grains has various disadvantages, such as a deterioration in granularity, increase in thickness of the emulsion layer, and deterioration in storability. Further, increasing sensitivity by using couplers with higher activities or accelerated development is disadvantgeous since these techniques not only are accompanied by a marked deterioration in granularity but also do not give rise to much of a contribution in sensitivity. With regard to acceleration of development, various development accelerators, including hydrazine compounds, have hitherto been incorporated into a silver halide emulsion layer or a developing solution therefor mainly for black-and-white photographic light-sensitive materials. However, in most cases, the incorporation of development accelerators into an emulsion layer or a developing solution is accompanied by disadvantages such as an increase in fog and a deterioration in granularity, and hence are impractical.
Further, couplers which release development accelerators or fogging agents in an imagewise manner have also been proposed. For example, couplers releasing thiocyanic acid ions which accelerate solution physical development are disclosed in U.S. Pat. Nos. 3,214,377 and 3,253,924, Japanese Patent Application (OPI) No. 17437/76 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), etc.; also, couplers releasing hydroquinone or aminophenol developing agents are described in Japanese Patent Application (OPI) No. 138636/82. However, the development accelerating function or the fogging function of these releasable groups is small and thus the effects obtained are very small even if couplers releasing these groups are used in a large amount. In order to overcome these problems, couplers releasing acyl hydrazines as described in U.S. Pat. No. 4,390,618 and couplers releasing thiocarbonyl compounds as described in Japanese Patent Application No. 161515/82 (corresponding to U.S. patent application Ser. No. 532,631 filed on Sept. 15, 1983 and West German Patent Application P No. 33 33 355.6 filed on Sept. 15, 1983) have been proposed and thus high sensitivity has been achieved with these couplers.
It has been found, however, that these couplers have the disadvantage in that color mixing occurs and the color reproducibility is degraded since the releasable groups diffuse into other layers which are sensitive to light in a different region in the spectrum to cause develpment acceleration or to form fog due to their high diffusibility, when they are incorporated into a certain light-sensitive layer which is sensitive to light of a specific region of the spectrum. It has also been found that these couplers have the disadvantage in that the granularity is deteriorated in comparison with cases wherein these couplers are not used to increase the sensitivity since the releasable groups freely diffuse in the layer to be added or into a layer which is sensitive to light of the same region in the spectrum causing development acceleration or forming fog even when they do not diffuse into the layers sensitive to light of a different region.
On the other hand, other techniques for increasing sensitivity based on the investigation on layer structure of photographic light-sensitive materials have been proposed. For example, a method to obtain high sensitivity in an emulsion layer by providing a layer containing fine reflective particles beneath the emulsion layer to utilize the light scattering function due to these fine particles as described in Research Disclosure, Vol. 134, page 47, No. 13452 (1975). This method is in fact effective to increase the sensitivity. However, the increase in sensitivity in this method is at most twice due to only the light scattering effect and the method is accompanied by difficulty in practical use in that the granularity is markedly deteriorated as the sensitivity is increased.